1. Field of Invention
The present invention relates generally to devices for protecting electronic circuits from excessive electrical surges, and more particularly to spark gaps implemented in conjunction with substrates such as, for example, printed circuit boards.
2. Background Art
A spark gap is a device that protects electrical systems and circuits from voltages that exceed the limits designed for such systems and circuits. This is referred to as “overvoltage” protection. Excessive voltages may be caused by lightning strikes, electric power transients or spikes, and electrostatic discharges. A spark gap accomplishes overvoltage protection by employing a gap containing a dielectric medium (e.g., air) between two electrically isolated conductors. The gap has a predefined width for maintaining electrical isolation until a threshold voltage differential develops between the two conductors. Upon reaching this threshold (or “breakdown voltage”), the dielectric strength of the gap's dielectric medium is exceeded, producing an arc and a discharge current across the gap. This causes the voltage differential between the conductors to decrease until the current extinguishes and the voltage differential drops below the threshold, at which time, the two conductors again become electrically isolated. Effectively, the spark gap limits the level of the voltage differential between the conductors. Typically, one of the conductors is at ground potential, and the discharge current flows to ground. The threshold or breakdown voltage is determined by the width of the gap and the dielectric strength of the dielectric medium in the gap.
For some time now, spark gaps have been employed on or in connection with printed circuit (PC) boards to provide overvoltage protection for delicate electronic circuitry mounted on the boards. Generally, such spark gaps (“PC board spark gaps”) are connected between a signal path and a ground path. The signal path may carry either information signals or power signals, or both. Typically, the PC board spark gap takes the form of opposing, space apart, conductive traces or sets of traces etched or deposited on the surface of the PC board. One of the traces or sets of traces are connected to the signal path and the other one of the traces or sets of traces is connected to the ground path. Examples of such PC board spark gaps are disclosed in the following patents: U.S. Pat. No. 7,126,804 to James et al.; U.S. Pat. No. 6,930,872 to Palinkas et al.; U.S. Pat. No. 6,600,642 to Karnes; U.S. Pat. No. 6,510,034 to Palinkas et al.; U.S. Pat. No. 6,059,983 to Noble; U.S. Pat. No. 5,969,924 to Noble; and UK Patent 2,053,579 to Griffiths et al.
PC board spark gaps that have a gap defined by conductive traces etched or deposited on the surface of the PC board have several drawbacks. First, the traces tend to degrade at the gap, due to vaporization caused by the energy from the arc discharges. The result is that the gap becomes wider, causing the threshold or breakdown voltage to increase. If the threshold increases too much, then the spark gap will not protect the circuitry on the PC board. In some cases, vaporization of the traces may leave behind carbon tracks on the surface of the board, which could cause sustained arcing or undesirable leakage currents. Second, in an effort to make PC board spark gaps more durable, the traces or trace features have been enlarged or multiplied (redundancy). This strategy necessarily requires greater surface area on the boards, leaving less area for the circuitry. Third, the precision of the gap width (and therefore the voltage threshold) is dependent upon, and thus limited by, PC board etching tolerances and errors.
Attempts have been made to overcome the degradation of printed circuit traces of PC board spark gaps. For instance, the traces have been made thicker and slots have been cut into or through the PC board below the gap; see, e.g., the Background discussion in U.S. Pat. No. 5,933,307 to West. West further proposes to use the conductive ends of surface mount devices (such as jumper or resistor SMDs) to establish the gap above the surface of the PC board. West also discloses a slot through the board, below the gap (see West FIGS. 5A & 5B). Similarly, U.S. Pat. No. 4,322,777 to Ueta et al. proposes to use copper balls soldered to opposing traces to establish the gap and protect the traces from vaporization (see, e.g., FIG. 2B, 4B, 5B, 6B, 7B, 8B, 9B, 10B, 11B, & 12B). Ueta et al. also discloses the use of slots into and through the PC board below the spark gap (see, e.g., FIGS. 4A, 4B, 6A & 6B). While these approaches may have met with some success, they require the mounting of additional parts on the board. In addition, they require solder joints to hold the parts in place on the conductive traces, which may raise a concern about the reliability of the joints after experiencing one or more arc discharges.
Another approach to the problem of creating a durable spark gap is to utilize features of the PC board other than the conductive traces. One such feature is the plated-through hole or via, i.e., a plated hole which extends through the PC board and is usually in electrical contact with conductive traces on one or both mounting surfaces of the PC board. In one type of embodiment, the plated-through hole functions directly as one of the electrodes of the spark gap. See, for example, the following patents: U.S. Pat. No. 6,560,087 to Zennamo, Jr. et al. (FIG. 5); and U.S. Pat. No. 4,160,210 to Molinari (FIG. 2). In these examples, the spark gap is implemented above the surface of the PC board. In another type of embodiment, the plated-through hole operates as a conductor to connect a spark gap electrode (on one surface of the board) to a ground contact (on the other surface of the board). See, for example, the following patents: U.S. Pat. No. 6,628,498 to Whitney et al. (FIG. 12); U.S. Pat. No. 6,285,535 to Nakamura (FIG. 3); and U.S. Pat. No. 6,172,590 to Shrier et al. (FIGS. 11-17). In Nakamura, an alumina (i.e., aluminum oxide) substrate is used and the plated-through hole is filled with a conductor. Shrier et al. also suggests an aluminum oxide substrate. Whitney et al. suggests that plated-through holes (vias) may be filled with plating material and that such holes may function as heat pipes or heat sinks In a further type of embodiment, the spark gap electrodes are wires placed across the surface of an aluminum oxide substrate and anchored in holes that are surrounded by metalized areas. See, e.g., U.S. Pat. No. 4,620,126 to Manske (FIG. 1). In these various embodiments, the concept of employing a plated-through hole or holes to define the gap of a spark gap (and advantages flowing from that) have been overlooked.
An effort to implement a spark gap using two plated-through holes embedded in a substrate is disclosed in the following patents: U.S. Pat. No. 5,714,794 to Tsuyama et al. and U.S. Pat. No. 7,161,784 to Cheung et al. In the Tsuyama et al. embodiments, etched traces (150), connected to the plated-through holes, establish the gaps of the spark gaps (see, e.g., FIGS. 15(a)-15(g)). In Cheung et al., multi-layered PC boards have spark gaps implemented with conductive traces and plated-through holes or cores (see FIGS. 9-11, & 13; cols. 4-5). Like Tsuyama et al., the conductive traces in Cheung et al. establish the gap of the spark gap, rather than the plated-through holes. In Cheung et al., the traces may be on the same surface (as in FIG. 9) or on different (overlapping) surfaces (as in FIGS. 11 & 13). Again, the concept and advantages of employing plated-through holes to establish the gap of a spark gap are not disclosed.
Spark gaps have been used to protect electronic filter circuits mounted on PC boards, such as those disclosed in the following U.S. patents: U.S. Pat. No. 7,126,804 to James et al.; U.S. Pat. No. 6,930,872 to Palinkas et al.; U.S. Pat. No. 6,560,087 to Zennamo, Jr. et al.; and U.S. Pat. No. 6,510,034 to Palinkas et al. These patents were previously cited and discussed above.